The adCenter blog announced last week that they will be expanding the content network, to allow more Microsoft adCenter advertisers to bid in the content network.
The thing is, if you get a notification of your account being upgraded into this test, you will have to manually opt out, if you do not want to participate.
Your campaigns will automatically be upgraded to include distribution on content pages within the Microsoft network.
If you want to participate in the test, US advertisers can requested an invite here.
If you don't, and you are upgraded, make sure to disable it within your account.
This does not mean Microsoft is accepting publishers like Google's AdSense or Yahoo!'s YPN program. It means Microsoft is accepting invites from advertisers to advertiser on their content network.